As a conventional technique related to a lubrication structure for a transmission, for example, Japanese Patent Laying-Open No. 05-033853 discloses an oil regulating apparatus preventing increase in friction or air suction by an oil pump caused by oil level change depending on an oil temperature. In the oil regulating apparatus disclosed in this document, a circulation system for an automatic transmission having an oil pan and an oil cooler includes an oil flow distributing portion having a distributing valve. The circulation system for an automatic transmission is connected to a reservoir tank having a return valve via the oil flow distributing portion. When the oil temperature is high, the oil is moved from the oil pan to the reservoir tank by means of the distributing valve. Meanwhile, when the oil temperature is low, the oil is returned from the reservoir tank to the oil pan by means of the return valve.
In addition, Japanese Patent Laying-Open No. 07-280181 discloses a breather apparatus aiming to improve assembly of a transmission and to efficiently recover the oil. The breather apparatus disclosed in this document includes a gear chamber and a coupled chamber communicating to the gear chamber through a communicating hole. The breather apparatus further includes a catch tank connected to the coupled chamber through a breather hose and located opposite to the gear chamber with the coupled chamber being interposed.
In the oil regulating apparatus disclosed in Japanese Patent Laying-Open No. 05-033853, in order to allow drive of the distributing valve based on the oil temperature, an oil temperature sensor for detecting an oil temperature or a controller for properly actuating the distributing valve based on an electric signal output from the oil temperature sensor should be provided. Accordingly, increase in the cost for parts or increase in the number of attachment steps may be caused. In addition, if the oil regulating apparatus is incorporated in a vehicle, such an apparatus is disadvantageous also from the viewpoint of weight of the vehicle.